Fig. 4. Overview of the NICE-OHMS experiments performed for H216O.
a NICE-OHMS spectrum of the (0 4 0)88,1/0 ← (0 0 0)77,0/1 doublet. The scanning frequency, f, is shifted with the line position of the ortho transition, fortho = 218,444,703.636 MHz (separated by 1.5 MHz from the para line). As clear from this spectral recording, the ortho line exhibits a three times larger intensity than its para counterpart, which is due to the nuclear-spin degeneracy factors and the nearly equal Einstein A coefficients of the lines. Note that the assignment of this doublet is interchanged in HITRAN20161. b Measured linewidth (half width at half maximum, HWHM) of the saturated resonance as a function of intracavity intensity for the (0 0 2)22,1/0 ← (0 0 0)33,0/1 ortho–para doublet. The pressure-free wavenumbers of these two lines are determined to be 7286.715 825 626(60) (ortho) and 7285.044 729 204(63) cm−1 (para). c Pressure-shift study of the saturated absorption line centers for some H216O lines with their assignments. The error bars represent one standard deviation. Source data are provided as a Source Data file.